The adoption, fostered by The Alliance for Children’s Rights and Westside Children’s Center, was finalized in Los Angeles on Wednesday, the Sex and the City actor, who currently stars on the USA Network's White Collar, tells PEOPLE exclusively.

"From the first time I met him, I said, 'That's my kid,' " says Garson, 45. His biggest challenge so far as a single parent? Helping Nathen with his homework.

"He’s in third grade, and I'm in the third grade," jokes Garson, who reprises his role as Stanford Blatch – the best guy pal of Carrie Bradshaw (Sarah Jessica Parker) – in the upcoming Sex and the City 2, which hits movie theaters May 28. – Monica Rizzo